This invention relates to bottles and similar articles blow molded from a thermoplastic material and more particularly to an improved method for reinforcing a bottle blow molded from a thermoplastic material. Many hollow articles, such as bottles, are blow molded from thermoplastic material. Such bottles commonly are used as packages for cosmetics and liquid food items which are either at atmospheric pressure or pressurized with a compressed gas. It is generally desirable to maximize the strength of bottles of this type for various reasons. When a liquid contained within a bottle is under pressure, bottles strength is important to prevent the bottle from rupturing. The bottle strength is also important since increasing the strength of a material from which a bottle is blown permits reducing the wall thickness without a loss in strength of the bottle. This in turn reduces the cost of the bottle and also reduces its weight when the bottle is shipped from the bottle manufacturer to a product manufacturer which fills the bottle and subsequent shipment of the filled bottle to the ultimate consumer. By modifying the design of a bottle to achieve a maximum strength, it is also possible to improve the appearance of the bottle and to to add gripping strength to the bottle to prevent accidental dropping.
Various prior art methods have been used for increasing the strength of bottles blow molded from thermoplastic materials, and particularly synthetic resinous materials. One method involves adding an impact modifier to the base resin from which the bottle is blown. Still another method for increasing the strength consists of increasing the wall thickness, which in turn provides the disadvantages of an increased cost and an increased bottle weight. The strength of a blown bottle has also been increased through design techniques which involve shaping the bottle to eliminate weak spots, and in some cases, adding reinforcement ribs to either the interior or exterior surfaces of the bottle. Still another method for increasing the strength of a bottle consists of carefully controlling the blowing techniques to biaxially orient the synthetic resinous material from which the bottle is manufactured. However, biaxial orientation is achieved only through careful control of the thermal history of the thermoplastic material from which the bottle was blown. If the thermal history is not carefully controlled, variations will occur in the degree of orientation of blown bottles and, as a consequence, the strength of the bottles will vary.